Despite going a goal down in the 13th minute, Ben Coulter nodding home at a far post, Grouville were level a few minutes later when Saints defender Richard Herpe put the ball into his own goal attempting to cut out a cross by Chris Carter.
With chances going astray at both ends it was Grouville who eventually nudged ahead, eleven minutes after the resumption, with a sublime goal worthy of winning any game.
From 25 yards out, Luc Le Mi?re drilled a fantastic left-footed strike into a top corner to put the Jersey side 2-1 up.
With the strong wind at their backs Grouville, for whom defender Craig Northedge was excellent, dominated the closing stages.
Le Mi?re lobbed goalkeeper Nathan Pattimore for his second and Grouville’s third before Alex Jeffries was denied by a great save from Pattimore.
Grouville play holders North in the final after North defeated Belgraves 2-1 on Saturday.
Grouville: Casey Hickling; Rob Cameron, David Syvret, Russell Le Feuvre, Craig Northedge, John Kiersey, Chris Carter, Marc Nightingale, Luc Le Mi?re, Alex Jefferies, Peter Warrell.